1. Field of the Invention
The present invention relates to an apparatus for controlling the electroplating of a strip being passed sequentially through a plurality of plating cells.
2. Description of the Prior Art
Prior art apparatus for controlling electroplating of a continuous strip through a plurality of plating cells has included automatic control circuitry for controlling the total current applied to the plurality of cells as a function of a measured speed of the strip in order to maintain an even plating thickness on the strip when the line speed increases or decreases. A total current per unit speed criterion is initially set based upon one or more factors such as the desired plating thickness, electrode efficiency and the width of the strip. A desired total plating current value is calculated by multiplying the total current per unit speed criterion by the measured speed of the strip. The actual total plating current is controlled by comparing a measured total plating current value with the calculated desired total plating current value and controlling the plating current to maintain the measured value equal to the calculated value.
FIG. 1 is a block diagram of one such conventional plating current control apparatus. A strip 1 is moved in the direction indicated by the arrow successively through plating cells 3a, 3b, 3c and 3d by a conventional drive mechanism (not shown) for electroplating a desired thickness on the strip. Sensors, such as current sensing resistive shunts 4a, 4b, 4c and 4d, are connected in series with the plating current lines from respective thyristor control circuits 5a, 5b, 5c and 5d to the respective plating cells. Controllers 6a, 6b, 6c and 6d have first inputs connected to outputs of the respective sensors 4a, 4b, 4c and 4dhave second inputs connected to outputs oil respective distributors 7a, 7b, 7c and 7d, and have outputs operatively connected to the respective thyristor circuits 5a, 5b, 5c and 5d. The controllers are designed to operate the thyrister circuits so that the plating current to each cell is proportional to the input voltage from the corresponding distributor, i.e., the plating current to each cell is increased when the voltage from the corresponding sensor is less than the voltage from the corresponding distributor, and is decreased when the voltage from the corresponding sensor is greater than the voltage from the corresponding distributor. An adder 8 has inputs connected to the outputs of the respective sensors 4a, 4b, 4c and 4d and is designed to produce an output which is the sum of the voltages from the sensors. The output of adder 8 is connected to one input of an adder 10a which subtracts the sum of the sensor voltages from a voltage applied to a second input of the adder 10a by an arithmetic circuit 10. The output of adder 10a is connected to the input of a PI controller 9 which has an output connected to inputs of the distributors 7a, 7b, 7c and 7d. The PI controller is designed to produce an output voltage which is the integral of its input so that when the output of adder 8 is less than the output of arithmetic circuit 10, the output of PI controller 9 is increased, and when the output of adder 8 is greater than the output of arithmetic circuit 10, the output of PI controller 9 is decreased. The distributors 7a, 7b, 7c and 7d are conventional multiplier circuits set to divide the input voltage from PI controller 9 by n, where n is the number of plating cells. The arithmetic circuit 10 has inputs from a current criterion circuit 11 and a speed sensor 2 mechanically coupled to a wheel engaging the strip 1. The current criterion circuit 11 is designed to produce a set voltage representing total current per unit speed criterion, and the arithmetic circuit 10 is designed to produce the product, i.e., multiple, of the outputs from the current criterion circuit 11 and the line speed sensor 2 so that the output of the arithmetic circuit 10 is proportional to the total plating current required to plate a desired thickness on the strip 1 at the measured line speed.
In operation of the apparatus of FIG. 1, the voltage output of PI controller 9 increases and decreases in accordance with increases and decreases in the speed of the strip 1 through the plating cells 3a-3d so as to maintain the production of a uniform plating thickness on the strip 1 during variations in the speed of the strip 1. The input voltage to each of the controllers 6a, 6b, 6c and 6d from the corresponding distributor is 1/n times the output of the PI controller 9 so that each controller 6a, 6b, 6c and 6d operates each thyristor power control circuit 5a, 5b, 5c and 5d to maintain the plating current in each cell directly proportional to the line speed.
Although the conventional plating control apparatus can maintain a total current through the plating cells which varies in accordance with line speed, there still exists deficiencies in the plating caused by line speed variations, such as a deficiency in the gloss of the plated surface, a deficiency in that variations in plating thickness resulting from a variation in electrode efficiency at different line speeds, and a deficiency in anti-corrosive characteristics of the plating. It has been proposed that these deficiencies can be reduced by maintaining a plating current density within a predetermined range. The prior art plating control apparatus cannot maintain a plating current density in the predetermined range while simultaneously controlling the total plating current in accordance with variations in the line speed.